High-aspect ratio nanostructures with widths between 10 nanometers (nm) and several micrometers and a height-to-width ratio in the range of 5-100 are becoming more widely used in many fields such as micro-electronics, nanotechnology, and diffractive optics. Like tiny sky-scrapers (e.g. Sear's Tower has an aspect ratio of roughly 15), stabilization of these thin and tall structures is a major engineering hurdle. An important challenge in producing these nanostructures is developing methods to enhance their stability without compromising their performance. This can sometimes be done easily, particularly in systems with no moving parts, such as semiconductor integrated circuits, but is often difficult in most other applications.
Because of the wavelengths involved, diffractive x-ray optics presents some of the greatest challenges. Two examples are gratings and zone plate lenses. These optical elements include repeating structures that block or phase-shift x-ray radiation. As with visible light optics, x-ray gratings are typically used to deflect an x-ray beam and spectrally separate polychromatic beams. A zone plate can be thought of as a circular grating, but with decreasing grating period towards the rim according to the relation rn2=nl fZ+an2l2. Such a zone plate behaves like a lens with focal length fZ=2rdr/l, where dr is the width of the outer-most and also the finest zone, and l is the wavelength. The diffraction limited resolution, according the Rayleigh criterion is simply d=1.22 dr, slightly larger than the outer zone width and independent of the wavelength.
Because of the short wavelength of x-ray radiation, feature sizes of x-ray optics must be very fine in order to diffract the beam to a large enough angle, but on the other hand the large penetration depth of x-ray radiation requires thick optics. As a consequence of these two properties, x-ray gratings and zone plates typically require high aspect ratios and the ratios increase when the x-ray energy is increased as the wavelength shortens and the penetration depth increases. For example, zone plates for focusing “soft” x rays with energy between 250 electron-Volts (eV) and 1000 eV typically have finest zone widths of 15-50 nm and aspect ratios of 3-10. For “hard” x rays with energies between 5 keV and 10 keV, the finest zone widths are 30-100 nm and aspect ratios are 15-30.